How can I validate user input to ensure it's the correct data type?

Question

Ryan McCombe · Accepted Answer

Validating user input is crucial for creating robust C++ programs. Here's a guide on how to ensure your input is of the correct data type:

Using Error States

When reading input with std::cin, it sets error flags if the input doesn't match the expected type. We can use these flags to validate input:

1#include <iostream>
2#include <limits>
3
4int main(){
5  int userAge;
6
7  while (true) {
8    std::cout << "Enter your age: ";
9    std::cin >> userAge;
10
11    if (std::cin.fail()) {
12      std::cin.clear(); // Clear error flags
13      std::cin.ignore(
14        std::numeric_limits<
15          std::streamsize>::max(),
16        '\n'); 
17      std::cout
18        << "Invalid input. Please enter a number.\n";
19    } else if (userAge <= 0 || userAge > 120) {
20      std::cout <<
21        "Age must be between 1 and 120.\n";
22    } else {
23      break; // Valid input, exit loop
24    }
25  }
26
27  std::cout << "Your age is: " << userAge;
28}

1Enter your age: hi
2Invalid input. Please enter a number.
3Enter your age: 25
4Your age is: 25

In this example, we use a while loop to repeatedly ask for input until it's valid. The std::cin.fail() check detects if the input wasn't an integer. If it fails, we clear the error flags and ignore the rest of the input line.

Using String Input and Conversion

Another approach is to read input as a string and then convert it:

1#include <iostream>
2#include <string>
3#include <stdexcept>
4
5int main(){
6  std::string input;
7  int userAge;
8
9  while (true) {
10    std::cout << "Enter your age: ";
11    std::getline(std::cin, input);
12
13    try {
14      userAge = std::stoi(input); 
15      if (userAge <= 0 || userAge > 120) {
16        throw std::out_of_range(
17          "Age out of valid range");
18      }
19      break; // Valid input, exit loop
20    }
21    catch (const std::invalid_argument&) {
22      std::cout
23        << "Invalid input. Please enter a number.\n";
24    } catch (const std::out_of_range&) {
25      std::cout <<
26        "Age must be between 1 and 120.\n";
27    }
28  }
29
30  std::cout << "Your age is: " << userAge;
31}

1Enter your age: hi
2Invalid input. Please enter a number.
3Enter your age: 25
4Your age is: 25

This method uses std::getline() to read the entire input line, then std::stoi() to convert it to an integer. We catch exceptions to handle invalid input or out-of-range values.

Remember, input validation is about more than just checking data types. Always consider the range of acceptable values and any other constraints specific to your program's needs.

User Input in the Terminal

Validating User Input in C++

Using Error States

Using String Input and Conversion

User Input in the Terminal

Intro to C++ Programming

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